This invention relates to a covered electrode for shielded metal arc welding, the covered electrode being of the class containing a relatively large amount of nickel and suitable for use in build-up welding for the purpose of providing hard surface to a metal body such as a metal die made of cast iron.
Currently it is prevailing to use cast iron as the material of relatively large-sized metal dies and fixtures for press-forming of steel sheet. For example, in the automobile industry very large-sized metal dies made of cast iron are used to form panels for car bodies. Of course it is necessary to provide hard surface to such metal dies by a certain kind of surface treatment. According to the recent trend in the design of car bodies, there is an increasing need of performing a very severe and precise stretch forming process to obtain a desirably shaped panel, and therefore greater attention is paid to the wear resistance and antifrictional property of the die face. As will be understood, wear of the die face results in lowering in the dimensional precision of the shaped panel and raises the need for considerable time and labour to repair the die. Where the antifrictional property of the die face is insufficient, the press-forming process is liable to suffer from seizing between the die face and the steel sheet brought into sliding contact with the die face and appearance of scratches on the shaped panel surface or unsatisfactory surface roughness of the shaped panel.
Hard chromium plating is an example of surface treatment techniques to provide a cast iron body with a hard, wear-resistant and fairly antifrictional coating layer. However, in the case of large-sized metal dies for severe stretch forming, the plated chromium layer is liable to peel off the cast iron surface during operation. Therefore, it has been the practice to form a hard coating layer by an overlaying welding or build-up welding process. However, hitherto developed iron base welding materials for this purpose are generally unsatisfactory primarily because of high probability of the occurrence of weld cracks. Besides, the deposited metal resulting from any of these welding materials is insufficient in its antifrictional property, so that the use of a metal die treated by this welding method results in considerable scratching of the shaped article. Furthermore, the treated die face itself undergoes considerable wear. Therefore, in this case it becomes necessary to use a highly effective lubricant in the press-forming process.
Recently developed nickel base welding materials are said to be suitable for use in build-up welding process for the purpose of overlaying cast iron bodies with a hard and antifrictional coating layer. It is true that when these nickel base welding materials are applied to metal dies the probability of scratching of the shaped articles can be greatly reduced. However, these nickel base welding materials are generally insufficient in the wear resistance of the deposited metal, so that the metal dies treated with these welding materials are short in service life and are liable to cause wrinkling of the shaped articles, particularly in the case of shaping large-sized panels.